(a) Field of the Invention
The present invention relates to a focal plane shutter of a type wherein said shutter is equipped with a front blade group and a rear blade group, and said rear blade group is returned to the start position (folded position) at the substantially final stage of the shutter cocking travel.
(b) Description of the Prior Art
In general focal plane shutters equipped with a front blade group and a rear blade group, the front and rear blade groups are brought to a position covering an exposure aperture (unfolded position) and a position not covering the exposure aperture (folded position) respectively when the shutter is to be cocked. During this cocking travel, the rear blade group must travel across the exposure aperture while keeping a required overlapped condition in order to prevent light leakage. The conventional focal plane shutter of a type so adapted as to perform the shutter cocking travel with a cam mechanism can satisfy this requirement by adopting a suitable cam shape design. However, in the focal plane shutter of the type, for example, proposed by Japanese Utility Model Preliminary Publication No. Sho 62-9241 wherein the shutter cocking travel is performed by use of rollers pivoted to a front blade actuating arm, a rear blade actuating arm and shutter cocking member respectively, a clearance may be formed between the front and rear blade groups at an ordinary cocking travel distance when the shutter has been cocked. Therefore, said type of focal plane shutter adopts a composition wherein a rear blade operating member in addition to said rear blade actuating arm to prevent the rear blade group from returning or moving to the position not covering the exposure aperture (folded position) until the shutter cocking operation completes or the front blade group reaches the position completely covering the exposure aperture (unfolded position) by locking said rear blade operating member, and when the front blade group reaches the position completely covering the exposure aperture, the rear blade operating member is unlocked by the rear blade operating member and allowed to move speedly to the start position (folded position).
In the latter type of focal plane shutter, however, wherein the rear blade group has a relatively long travel from its start to stop after completing the shutter cocking operation, the rear blade group travels with inertia and bounds rather severely at its stop position. Therefore, the focal plane shutter of this type has a drawback that the bound makes exposure time unstable or causes hindrance in successive photographing.
Further, in the focal plane shutter of said type wherein the front and rear blade groups are brought to the unfolded position and folded position respectively when the shutter is to be cocked, it is general to release the movable contact member from the fixed contact in synchronization with the operation of the rear blade locking lever or rear blade actuating lever between the movable contact member for synchroflashing and the fixed contact. The focal plane shutter having this composition allows a trouble of erroneous flashing due to accidental contact between the movable contact member and fixed contact unless sufficient consideration is taken to assure proper synchronization between the cocking travel of the front blade actuating lever and the rear blade locking lever or rear blade actuating lever for the shutter cocking operation.